Wavelength division multiplexing (WDM) is an efficient way of multiplying the capacity of optical fibre. In wavelength division multiplexing, several independent transmitter-receiver pairs use the same fibre. FIGS. 1a and 1b illustrate the principle of wavelength division multiplexing, using as an example a system having four parallel transmitter-receiver pairs. Each of the four information sources (not shown in the figure) modulates one of four optical transmitters, each of which generates light at a different wavelength (λ1 . . . λ4) As will be seen from FIG. 1a, the modulation bandwidth of each source is smaller than the distance between the wavelengths, and thus the spectra of the modulated signals do not overlap. The signals generated by the transmitters are combined onto the same optical fibre OF in a WDM multiplexer WDM1, which is a fully optical (and often passive) component. At the opposite end of the fibre, a WDM demultiplexer WDM2, which is also a fully optical (and often passive) component, separates the different spectral components of the combined signal from one another. Each of these signals is detected at a discrete receiver. Hence, a narrow wavelength window is assigned for the use of each signal in a given wavelength range. A typical practical example might be a system where the signals are in the 1550 nm wavelength range for example in such a way that the first signal is at the wavelength 1544 nm, the second signal at the wavelength 1548 nm, the third signal at the wavelength 1552 nm and the fourth signal at the wavelength 1556 nm. Nowadays a multiple of 100 GHz (approx. 0.8 nm) is becoming the de facto standard for the distance between wavelengths.
In order to enable a given wavelength channel to be branched off a transmission link using wavelength division multiplexing or a given wavelength channel to be added thereinto, an element called an add/drop filter has been developed. The task of an optical add/drop filter is (1) to direct off a selected narrow-band channel (wavelength) from the optical aggregate signal that passes in the fibre incoming to the filter (drop function) and/or (2) to add to the fibre outgoing from the filter a narrow-band channel (add function). The signals (wavelengths) that have not been selected for dropping pass through the add/drop element from the incoming fibre to the outgoing fibre. Thus a desired narrow-band channel is added or dropped in the filter without otherwise affecting the spectrum of the optical aggregate signal in any way (providing that free wavelengths are available in the fibre involved).
FIG. 2 illustrates the structure of a known three-port add/drop filter OADE. References L1 . . . L3 denote port-specific lenses and reference F an interference filter. The incoming fibre is connected to port A, through which a number of wavelength channels (λ1 . . . λ4) arrive. One of the wavelength channels (in this example λ1) passes through the interference filter F (and lenses L1 and L2) to port B. The remaining wavelength channels also pass through lens L1 but are reflected from the interference filter (through lens L3) to port C. The wavelength of the channel entering port B is a fixed, filter-specific constant.
The filter operates in both directions, and hence the adding function is achieved in such a way that the existing channels are fed to port C and a channel to be added to port B, and hence all channels are obtained from port A.
A three-port filter of the kind described above is manufactured for example by Optical Corporation of America, U.S.A.
The existing optical telecommunications systems based on wavelength division multiplexing have mostly been point-to-point systems used on high-capacity long-distance connections (trunk lines). However, optical transmission technology is being continually developed to be able to implement the lowest layers of broadband network architectures as fully optical systems, which would make it possible to handle the transmission of high-capacity information flows by purely optical means (i.e. using optical cross-connection and routing).
After the point-to-point systems, optical networks allowing add/drop functions, such as ring networks, will be the next step in this development. To make it possible to configure this type of network flexibly in response to traffic loads, add/drop filters nave been upgraded to network elements allowing free selection of the wavelengths to be dropped and/or added. This type of add/drop device is, thus, a network element that can be configured to allow free selection of the wavelengths to be dropped/added.
The following section will provide a brief description of a typical ring network with reference to FIG. 3. In the example shown in FIG. 3, the network is used for transmitting SDH (Synchronous Digital Hierarchy) signals, but the type of signal carried by each wavelength can, naturally, vary. The signal may be any analog or digital signal, such as a PDH (Plesiochronous Digital Hierarchy) signal or an ATM (Asynchronous Transfer Mode) signal.
In this example, the ring network features four nodes consisting of the add/drop devices OADM1 to OADM4. For both directions of transmission, there is a dedicated ring. Between the nodes, there is an optical transmission connection consisting of optical fibres OF, and wavelength division multiplexing, as described above, is used on each one-way connection between two nodes. In the example, four wavelengths (λ1 . . . λ4) and one management wavelength (λm) are used, but it is understood that the number of wavelengths used in the network may vary and be even higher. Each add/drop device may have an interface to the control system ONC of the device and/or of the entire optical network, which allows all the add/drop devices in the network to be configured as desired.
The equipment in an optical network can be integrated with SDH equipment and the optical control system may be integrated with the control system of the SDH equipment. The optical signal from the SDH device is connected to the ring network at the selected wavelength. A desired number of wavelengths is defined for node-to-node connections in accordance with the traffic needs. In other words, the routing configurations corresponding to the various wavelengths can be changed in response to traffic load. Using the management signal carried by the management wavelengths, it is possible to configure each individual node to drop/add the desired wavelengths. As shown in the figure at node OADM1, there may be a dedicated drop element MCD for node management wavelength in both directions to drop the control channel at the management wavelength λm to the node control unit CU, which, then, converts the signal into an electric signal and controls the add/drop part AD of the node as indicated by the management signal to drop the desired channels into the SDH equipment and to add the desired wavelengths from the SDH equipment. In addition to the drop element, there is an add element MCA for both transmission directions to add the management channel from the control unit CU to the signal being transmitted by the node.
Node management can also be effected directly by means of a local control interface (optical or electric) to control each node individually through the local interface; node management can also be effected using both a local control interface and the network management channel, so that a signal can be connected to the network management channel via the local control interface.
Each wavelength can, for example, be used to carry an STM−N (Synchronous Transport Module) signal, such as STM−16 (N=16). In the example of FIG. 3, it has been simply presumed that wavelength λ1, corresponds to SDH device SDH1, wavelength corresponds to SDH device SDH2, wavelength λ3 corresponds to SDH device SDH3 and wavelength λ4 corresponds to SDH device SDH4.
One important consideration besides the cost-effectiveness of the network is its reliability in operation. To ensure this, ring networks usually feature a protection system with optical switches to make it possible to create the required connection via the remaining ring in the event that a connection between two nodes fails.
FIGS. 4 and 5 illustrate a typical protection system. Network nodes N1-N4 are inter-linked by two separate rings: working WR and protection ring PR. Normally, when the network is operating faultlessly, the signal is carried in one direction along working ring WR and the desired signals are dropped/added at the nodes (FIG. 4). If a fault occurs, when, for example, the connection between two nodes fails, as exemplified by nodes N2 and N3 in FIG. 5, the signal is switched from the working ring to the protection ring at the node (N2) in the working ring located before the failure point, and from the protection ring to the working ring at the node (N3) located after the failure point. Thus, the signal travels along the working ring in the forward direction and along the protection ring in the return direction. The arrows in the figures show the route of one wavelength channel from node N1 to node N3 in both cases.
With the increased use of ring networks, optical telecommunications networks will increasingly be implemented as networks covering a more limited geographical area. In the next phase, the networks will typically be regional networks with a diameter of 10 to 100 km. Since the number of nodes and different interfaces in such networks will increase dramatically, it is of growing importance that the network and protection system can be built using a minimum number of components. This is of special importance in the initial phase because the WDM technology is still expensive for the user. Moreover, it is vital that all the components can be manufactured using existing and economically feasible technology. A drawback of the protection system described above, for example, is that if the add/drop devices are only used in the working ring, the signal path for the completion of the required functions becomes rather long. This is due to the fact that at some nodes channel dropping/addition will not take place until the signal returns along the working ring (WR). As a result, extra amplification may have to be provided in the network just to ensure that the protection system will work.
Another important point in the interest of cost-effectiveness is that the node transmitters and receivers can be implemented at as low a cost as possible. As far as optical receivers are concerned, they are capable of receiving several wavelengths. However, transmitters, for example, usually operate only at one wavelength. Transmitters operating at multiple wavelengths have been developed, but they are clearly more complicated to manufacture and manage than single-wavelength transmitters. Therefore, it would be useful if networks where a connection can be established between any two nodes (such as in a total mesh network), would, at least initially, be able to manage with one single-wavelength transmitter at each node (excluding the spare transmitter).